As well known in the art, passively discharging and water-saving toilet bowls, which have been developed as alternatives to siphon-type toilet bowls, have no problems removing stool using half of the amount of water required by conventional toilet bowls based on the siphon principle. In addition, passively discharging and water-saving toilet bowls do not employ compulsory discharging manners but take advantage of gravity to remove the stool so as to generate no flushing noise. For this reason, they are attracting intensive attention as next-generation toilet bowls.
Such a passively discharging and water-saving toilet bowl, as seen in FIGS. 1 to 3, comprises a body 1 in which a slanted water drain path 1a is provided at the rear so as to induce the passive discharge of the stool; a water reservoir 2 for supplying water to the body; an operation unit 3, installed in the reservoir, for removing the stool a septic tank 4, installed underground, for storing the discharged stool; and a stool-discharging piston 6, provided within the stool discharging pipe 5 that communicates with the body and the septic tank and selectively opens the body and the septic tank.
The operation unit 3 includes a first link 3a which can rotate in a seesaw manner, is connected to a manipulation lever 7 and is supported to an internal wall of the water reservoir 2 at its one terminal portion, and a second link 3b which can rotate in a seesaw manner and is supported to an internal wall of the water reservoir 2, with its one terminal portion partially overlying the first link 3a. Because they are both connected to one end of the second link 3b, the stool-discharging piston 6 and a flap valve 8 move up and down according to the position of the end of the second link 3b. 
However, such a conventional water-saving toilet bowl has a critical drawback of operating faultily. Although enjoying the advantages of flushing quietly and having low water consumption, the conventional water-saving toilet often shows inadequate operation such that water overflows the body 1 upon flushing, or the stool is not discharged.
Fundamentally, these problems with the basic functionality are based on a subsidiary water tank 9. When the second link 3b is slanted downwards, the subsidiary water tank 9 has to function to maintain the slanted state for a predetermined time. In most cases of operational problems, the subsidiary water tank 9 does not perform this function, so that the second link does not remain in the slanted state for a sufficient amount of time, but returns back to the original state.
Accordingly, the stool-discharging piston 6 closes in advance of complete removal of the stool. In the meantime, while the flap valve 8 is caused to remain open by buoyancy, the stool-discharging pipe is closed. In this condition, water is continuously supplied from the reservoir to the inside of the body 1 upon flushing, so that overflow occurs.
Additionally, the subsidiary water tank 9 is not smoothly moved downwards upon flushing because of the resistance of the buoyancy exerted thereto. Hence, the manipulation lever 7 with which the subsidiary water tank 9 is forcibly moved down is not smoothly operated.
Through a wire W, the second link 3b is connected both to the stool-discharging piston 6 and to the flap valve 8. Since the wire W is at a slanted angle with respect to the direction of the force of the manipulation lever upon flushing, a larger force is needed than that required when it is in line with the direction. Thus, more force must be applied to the manipulation lever 7, reducing the lifespan of the parts used and the reliability of their operation.